(1) Field of the Invention
This invention relates to a resin composition, articles which is made of the composition, and a method for producing the same, wherein the resin composition excels in thermal resistance and workability in forming, and also especially excellent in adherence in metal plating and external appearance.
(2) Description of Prior Art
When the surfaces of resin articles are applied with metal plating, it is necessary that the properties of resin surfaces are suitable for receiving the treatment of metal plating. More particularly, the condition of the surface of resin must exhibit sufficient adherence to a plated layer (hereinafter referred to as "plating strength").
When metal plating is applied to the surfaces of formed articles which are made of wholly-aromatic thermotropic liquid crystal polyester resin, the articles to be plated are formed using a resin composition containing an inorganic filler that is soluble in an acid or an alkaline solution, and the surfaces of obtained resin articles are then etched with an acid or an alkali solution, thereby dissolving or modifying all of or a part of the inorganic filler existing in the surfaces of articles, and converting the surfaces of resin into a condition suitable for the metal plating. In this treatment, because the plated layers are caught by the surfaces of articles with the so-called anchor effect, it is also necessary for increasing the plating strength that the mechanical strength of the surfaces of articles is large enough.
However, owing to the higher melting points of wholly-aromatic liquid crystal polyester resins, they are liable to be hydrolyzed by coexisting moisture. This phenomenon is accelerated by the catalytic action of the contained acid- or alkali-soluble inorganic fillers themselves or other metallic components contained in the fillers. Furthermore, the reaction is sometimes caused to proceed by the acid which is produced by decomposition. As a result, decomposed products are often produced in wholly-aromatic liquid crystal polyester resins.
The coexisting moisture in a resin composition is derived from the one contained in inorganic fillers such as calcium carbonate or the one that is adsorbed by the resin itself. In the case of wholly-aromatic polyesters, the quantity of moisture that is adsorbed by the resin itself is very small, however, even though the quantity of moisture is trace, the reaction of hydrolysis proceeds rapidly because the melting point is high.
Accordingly, the liability to hydrolysis must always be taken into consideration in the case of wholly-aromatic polyester resin compositions containing inorganic fillers.
In the case of ordinary resins, molecules of molten resin are disposed at random in the molten state in heating. However, the thermotropic liquid crystal resin is characterized to exhibit the liquid crystal property when it is applied with shearing stress in a molten state. This fact means that the resin is easily oriented and is caused to flow in the molten state. Owing to this fact, in the case that less viscous substances such as decomposed products exist in a resin, they tend to be transferred to the surface portion of molten resin, that is, the boundary with wall surface.
As a result, the decomposed product is converged at surface portions. Even though the quantity of decomposed products is very small as compared with the quantity of the whole resin, the condition of surface is largely influenced by this convergence at the surface portion.
In an ordinary resin, for example, in a simple non-liquid crystal polyester resin, if it is of crystalline nature, the convergence of foreign substances at the surface portions of melt-molded articles is sometimes observed. (In the case of the non-liquid crystal resin, the foreign substances are considered to be low molecular weight resins.) However, in the case of these resins, the quantity of transfer is smaller, beyond comparison, than in the case of the liquid crystal polymer.
However, in the thermotropic liquid crystal polymer, the mass-transfer of less viscous substances such as decomposed products is caused to occur quite easily. In other words, in the case of the thermotropic liquid crystal polymer, foreign substances (decomposed products) are easily shifted to surface portions even when the quantity of the foreign substances is very small, as described in the foregoing paragraph. Therefore, it is considered that the influence of decomposed products on the surface portion is much larger beyond comparison than in the case of a non-liquid crystal resin, even when the quantity of decomposed products is quite small.
For example, it is described in European Patent Publication No. 311232 (A) relating to liquid crystal polyester resin composition for metal plating, in Comparative Example, that the composition containing calcium carbonate is not desirable (ibid., Example 4, Method for preparing test pieces; and Comparative Example 11).
In the same Comparative Example, after preparing pellets by melting and mixing a resin and an inorganic filler, the pellets are dried and test pieces are formed after the drying. In this preparation of pellets, the resin is melted by heating together with the filler. That is, because the resin is melted together with the filler in the presence of water, hydrolysis is already caused to occur in the resin. This hydrolyzed product is concentrated to the surfaces of formed articles of the resin. Therefore, it is supposed that the plating strength of resin articles is lowered. Once decomposed products are generated, it is considered natural that they produce undesirable influences on the metal plating process of formed articles unless they are removed from the polymer. In addition, such removal is not easy.
Incidentally, the foregoing hydrolyzed products in surface layer cannot be easily removed by the conventional pretreatment for metal plating process using treating solutions such as etching solutions, or organic solvents which are used by mixing with the etching solutions or used together. Even when the removal is possible, it is only limited to the case in which the hydrolysis proceeds to a considerable extent. On the other hand, in the case that the hydrolysis does not so proceed and some decomposed products are left unremoved or remained, it is considered that the remaining substances produce undesirable influences on the surfaces of formed articles in the process of metal plating. Therefore, it is inevitable to avoid the occurrence of hydrolysis.
Furthermore, the liquid crystal polyester resins are proposed for a fresh development of the next generation to use for electric and electronic parts and accessories. In these uses, the metal plating is an inevitable requirement.